The boat came with this little bitty Standard Horizon depth readout attached with velco to the helm. The transom mount transducer for it had been ripped off at some time in the past. I tried to order a replacement transducer from Jamestown Distributors. I must have gotten some numbers wrong, because what I actually purchased just showed up yesterday. It's much bigger than what I was expecting, with the wrong connector etc.

What I got is an Airmar Standard DST521 Depth/Speed/Temp Triducer Multisensor.

It would cost me $100 to return it, and I'd have to fill out a bunch of customs paperwork to export it.

So, I'm wondering, what if I bought the rest of the system to go with it. How can I find out what transceiver would go well with this transducer?

on the label I found another part number, which I think is Airmar. It's 31-492-1-01.
I wrote Jamestown, no joy. I ran the DST521 by Standard Horizon and see that it's the only transom mount ducer they make now. So I guess the one I was trying to replace is obsolete. It was half the size of this thing.

Std. Horizon lists several "fish finders" that use this, but they tend to be integrated GPS chart plotters. I'm interested in a stand alone depth system, if such a thing is still available. I tried running these numbers through the Airmar website, and just get "No results" for the search. It's not a P66, but seems to be a P58.

After much thought, I've decided to go with stand alone Chart Plotter, depth, radar, AIS, and radios instead of this all-in-one-box approach. So none of the Standard Horizon products are acceptable. They're all combined chart plotter/fish finders.

dang, looks like I screwed myself buying this ducer. All Std. Horizon sells are combined GPS chart plotters and fish finders. Pre-loaded GPS charts are pretty much junk outside the USA. all I need is the depth transceiver and a display.

You just want to use the depth part of this transducer - correct? If so, then 3 of those wires send the depth information that should match up with your Standard depth sounder. They will be the positive, negative and shield wires, but you will need to determine which ones those are on both ends.

Your instrument will not be able to use the speed, temp or one of the depth frequencies, but the remaining functionality should work fine with it.

Airmar should be able to help you with this, or they may have the wiring diagrams on their website.

I can't see where mounting location makes any practical difference. If it was mounted at the tip of the bow, that would only give one ~15' of warning before the keel. That is 2 seconds at 5kts. Typically, depth sounders are located at the front root of the keel, so there is no warning at all.

The OP has a Catalac 12 (big cat). I couldn't find a good image of it's bottom profile but draft is listed as 3' 1", presumably located in the middle, so if the OP wants to gunkhole or approach a beach nose-in... a forward location is definitely better than the transom.

Also, the transom-mount transducer is much more prone to damage or being snapped off than in-hull or thru-hull. Which is why the OP needs a new one.

I've also got a hull mounted transducer in one of the hulls. It's wired to the B&G Quad system, which is basically the wind system. The wind system doesn't work, since the lightning strike. I don't plan to replace the wind instruments. I just don't see the utility in knowing the wind velocity to a tenth of a knot or direction to a degree. Not at those prices. So when I eventually get around to removing the B&G stuff, it's going to leave three holes in the helm. Right now, one of those B&G boxes is the boats only depth display.

The transducer mounted at the transom had been ripped off at some point in the past. I was trying to just replace the Standard Horizon, transom mounted depth transducer. So I looked up "Standard Horizon transom mounted depth transducer" on the Jamestown website. They only list one. I checked The Standard Horizon website. They only list one, too. And since the entire world only lists ONE Standard Horizon transom mount transducer, I just assumed that it would be the one that I needed for my Standard Horizon depth system. I was wrong. They once offered a different one, obviously. But it's difficult to know this looking at photos on web sites. So I ordered the wrong one.

So, now I find myself with:
A hull mounted transducer hooked up to a B&G wind display that also has depth, which works except for the wind data, and an Airmar dual freq. transom mount transducer that presently has no display.

I don't think a 12 m catamaran is a big boat at all. In fact, it keeps getting smaller all the time.

I agree with Lake Effect above. Why bother with a transom mounted depth transducer? What's wrong with the hull mounted one you have now?

I would sell the transducer and either live with what you've got, or put in a new complete freestanding depth system, but mount it in the hull, not on the transom.

Or, alternatively, you could rethink your idea about obsessively keeping everything separate, and buy an NMEA2000 depth sounder, and an NMEA2000 display. Then it will be child's play to add wind to that -- you'll have two N2K displays and two N2K transducers on a super simple network which any child can install and configure, which can be expanded infinitely in the future should you so desire.